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4 Sep 2000

Volume 77, Issue 10, pp. 1413-1560

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High-performance all-polymer integrated circuits

G. H. Gelinck, T. C. T. Geuns, and D. M. de Leeuw

Appl. Phys. Lett. 77, 1487 (2000); http://dx.doi.org/10.1063/1.1290728 (3 pages) | Cited 232 times

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In this letter, we demonstrate the integration of all-polymer field-effect transistors in fully functional integrated circuits with operating frequencies of several kHz. One of the key items is an approach to incorporate low-Ohmic vertical interconnects compatible with an all-polymer approach. Inverters, NAND gates, and ring oscillators with transistor channel lengths down to 1 μm have been constructed. Inverters show voltage amplification at moderate biases and pentacene seven-stage ring oscillators show switching frequencies of a few kHz. The potential to realize large integrated circuits is demonstrated by a 15 bit code generator circuit using several hundreds of devices. The proposed concept was evaluated for three solution-processable organic semiconductors. © 2000 American Institute of Physics.
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85.30.Tv Field effect devices
85.40.Qx Microcircuit quality, noise, performance, and failure analysis
84.30.Sk Pulse and digital circuits
84.30.Ng Oscillators, pulse generators, and function generators
73.61.Ph Polymers; organic compounds

Experimental proof-of-principle investigation of enhanced Z3DT in (001) oriented Si/Ge superlattices

T. Koga, S. B. Cronin, M. S. Dresselhaus, J. L. Liu, and K. L. Wang

Appl. Phys. Lett. 77, 1490 (2000); http://dx.doi.org/10.1063/1.1308271 (3 pages) | Cited 54 times

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An experimental proof-of-principle of an enhanced Z3DT (thermoelectric figure of merit) is demonstrated using (001) oriented Si/Ge superlattices. The highest value of the experimental Z3DT at 300 K for a (001) oriented Si(20 Å)/Ge(20 Å) superlattice is 0.1 using κ = 5 Wm−1 K−1, for the in-plane thermal conductivity, which is a factor of seven enhancement relative to the estimated value of Z3DT = 0.014 for bulk Si. The good agreement between experiment and theory validates our modeling approach (denoted as “carrier pocket engineering”) to design superlattices with enhanced values of Z3DT. Proposals are made to enhance the experimental values of Z3DT for Si/Ge superlattices even further. © 2000 American Institute of Physics.
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73.61.Cw Elemental semiconductors
73.50.Lw Thermoelectric effects
72.20.Pa Thermoelectric and thermomagnetic effects
68.60.Dv Thermal stability; thermal effects
66.70.-f Nonelectronic thermal conduction and heat-pulse propagation in solids; thermal waves

Use of multiple electrical pulses to study charge transport in polymer light-emitting diodes

D. J. Pinner, R. H. Friend, and N. Tessler

Appl. Phys. Lett. 77, 1493 (2000); http://dx.doi.org/10.1063/1.1308275 (3 pages) | Cited 4 times

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The method of applying two consecutive voltage pulses is presented as a method to study the dynamics of injected charges in the “off-state,” i.e., after the removal of the applied voltage pulse. Pulses of different widths are used to selectively probe the dynamics of holes and electrons. The use of multiple (>2) pulses is also described as an additional carrier monitoring technique. © 2000 American Institute of Physics.
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85.60.Jb Light-emitting devices

Lateral epitaxy overgrowth of GaN with NH3 flow rate modulation

X. Zhang, P. D. Dapkus, and D. H. Rich

Appl. Phys. Lett. 77, 1496 (2000); http://dx.doi.org/10.1063/1.1308055 (3 pages) | Cited 16 times

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We demonstrate the effects of NH3 flow modulation on the lateral growth rate and morphology of GaN stripes employing lateral epitaxial overgrowth (LEO) by metalorganic chemical vapor deposition. The self-limiting growth mechanism, enhanced Ga diffusion on the (0001) plane, and Ga lateral supply are used to explain our observations. A lateral overgrowth rate to a vertical growth rate ratio of 2.1 and fully coalesced LEO GaN stripes after 1 h growth have been achieved. © 2000 American Institute of Physics.
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81.05.Ea III-V semiconductors
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
68.35.Fx Diffusion; interface formation

Hydrogen passivation of deep levels in n–GaN

A. Hierro, S. A. Ringel, M. Hansen, J. S. Speck, U. K. Mishra, and S. P. DenBaars

Appl. Phys. Lett. 77, 1499 (2000); http://dx.doi.org/10.1063/1.1290042 (3 pages) | Cited 29 times

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Differential postgrowth hydrogen passivation of deep levels in n–GaN grown by metal-organic chemical vapor deposition has been directly observed by means of both deep level transient spectroscopy and deep level optical spectroscopy. Two deep levels found at Ec−Et = 0.62 and 1.35 eV show strong H passivation effects, with their concentrations decreasing by a factor of ⩾30 and ∼14, respectively. The decrease in the 0.62 eV trap concentration together with its correlation with the presence of Mg in n–GaN is consistent with Mg–H complex formation. A band of closely spaced deep levels observed at Ec−Et = 2.64–2.80 eV narrows to Ec−Et = 2.74–2.80 eV after hydrogenation, consistent with hydrogen complexing with VGa3− defects as anticipated by earlier theoretical results. Finally, a deep level at Ec−Et = 3.22 eV likely related to background acceptors remains unaffected by hydrogen. © 2000 American Institute of Physics.
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71.55.Eq III-V semiconductors
81.65.Rv Passivation
81.05.Ea III-V semiconductors
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Giant magnetostriction in an ordered Fe3Pt single crystal exhibiting a martensitic transformation

Tomoyuki Kakeshita, Tetsuya Takeuchi, Takashi Fukuda, Masato Tsujiguchi, Toshio Saburi, Ryuichiro Oshima, and Shunsuke Muto

Appl. Phys. Lett. 77, 1502 (2000); http://dx.doi.org/10.1063/1.1290694 (3 pages) | Cited 112 times

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Magnetostriction measurements have been made in an ordered Fe3Pt single crystal with degree of order of about 0.8, which exhibits a cubic-tetragonal martensitic transformation at 97 K. The specimen was cooled down to 4.2 K without magnetic field, and then a magnetic field of 4 T is applied to the specimen along 〈001〉 at 4.2 K and removed. As a result, a reversible giant magnetostriction of about 0.5% is observed. This reversible magnetostriction will be caused by the rearrangement of crystallographic domains, being three times as large as that of Terfenol-D (Fe2DyxTb1−x: typical magnetostrictive materials). © 2000 American Institute of Physics.
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75.80.+q Magnetomechanical effects, magnetostriction
64.70.K- Solid-solid transitions
75.60.Ch Domain walls and domain structure
75.50.Bb Fe and its alloys
81.30.Kf Martensitic transformations

Determination of average demagnetizing fields in longitudinal magnetic recording using nanosecond field pulses

A. Moser, D. Weller, N. D. Rizzo, and T. J. Silva

Appl. Phys. Lett. 77, 1505 (2000); http://dx.doi.org/10.1063/1.1290688 (3 pages) | Cited 3 times

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The demagnetizing field across bit transitions in longitudinal recording media is measured quantitatively as a function of linear density using a quasi static write/read tester. Bit transitions with varying linear densities in the range 500–6000 flux changes per mm are recorded on CoCr10Ta4 media films. The media are deposited onto a coplanar waveguide structure, which is used to generate reversal field pulses of well-defined amplitude and 10 ns width to minimize thermally activated processes. The demagnetizing field is extracted from the measured reversal field, which is the sum of the external waveguide field and the internal, density-dependent demagnetizing field. The experimental results are qualitatively consistent with those predicted by a simple magnetostatic model that assumes finite transition widths. © 2000 American Institute of Physics.
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75.50.Ss Magnetic recording materials
75.70.Ak Magnetic properties of monolayers and thin films
75.50.Cc Other ferromagnetic metals and alloys
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects

Magnetotunneling in sputtered La0.7Ca0.3MnO3/Al2O3 multilayers

Y. Luo, A. Käufler, and K. Samwer

Appl. Phys. Lett. 77, 1508 (2000); http://dx.doi.org/10.1063/1.1290718 (3 pages) | Cited 14 times

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Magnetic multilayers in the form of [La0.7Ca0.3MnO3/Al2O3]n+La0.7Ca0.3MnO3 were sputtered to study their magnetotunneling effect. Structural analyses were performed by standard x-ray scattering, showing a well-defined superlattice. Both current perpendicular to plane (CPP) and current in plane (CIP) methods were applied for the magnetoresistance measurements. The CPP value gives the tunneling effect through the thin insulating layers and shows a large low-field effect of about 60% for the trilayer junction and 50% for the superlattice structure at 5 K, respectively. The CIP effect is about 30% (5 K) for all samples and is dominated here by the contribution of the intergrain tunneling effect within the La0.7Ca0.3MnO3 layers. © 2000 American Institute of Physics.
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75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
73.50.Jt Galvanomagnetic and other magnetotransport effects (including thermomagnetic effects)
75.45.+j Macroscopic quantum phenomena in magnetic systems
73.61.Ng Insulators

Scanning aperture photoemission microscopy for magnetic imaging

Gary M. McClelland and Charles T. Rettner

Appl. Phys. Lett. 77, 1511 (2000); http://dx.doi.org/10.1063/1.1290721 (3 pages)

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Magnetic imaging has been demonstrated by photoemission microscopy in which spatial resolution is achieved by scanning an aperture across the sample. To achieve magnetic contrast, the difference between photoemission current with right and left circularly polarized light is recorded while holding the aperture-surface distance to 30 nm. We used conical gold tips with apertures as small as 30 nm, drilled by a focused ion beam. Images of Co/Pt multilayer films show a magnetic contrast of ±2% and a resolution (25%–75% of a step) of 30 nm. Resolution better than 10 nm should be achievable with smaller apertures. With this imaging method, there is no magnetic interaction between sample and tip, and ultrafast time resolution should be achievable. © 2000 American Institute of Physics.
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07.55.-w Magnetic instruments and components
07.79.-v Scanning probe microscopes and components
79.60.-i Photoemission and photoelectron spectra
75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)

Sm–Co–Cu–Ti high-temperature permanent magnets

J. Zhou, R. Skomski, C. Chen, G. C. Hadjipanayis, and D. J. Sellmyer

Appl. Phys. Lett. 77, 1514 (2000); http://dx.doi.org/10.1063/1.1290719 (3 pages) | Cited 72 times

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A class of promising permanent-magnet materials with an appreciable high-temperature coercivity of 8.6 kOe at 500 °C is reported. The Sm–Co–Cu–Ti magnets are prepared by arc melting and require a suitable heat treatment. Magnetization measurements as a function of temperature and x-ray diffraction patterns indicate that the samples are two-phase mixtures of 2:17 and 1:5 structures. Depending on heat treatment and composition, some of the magnets exhibit a positive temperature coefficient of coercivity. The promising high-temperature behavior of the coercivity is ascribed to the temperature dependence of the domain-wall energy, which affects the curvature of the walls and the pinning behavior. © 2000 American Institute of Physics.
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75.50.Ww Permanent magnets
75.50.Cc Other ferromagnetic metals and alloys
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
81.40.Rs Electrical and magnetic properties related to treatment conditions
81.40.Gh Other heat and thermomechanical treatments
75.60.Ch Domain walls and domain structure

Anomalous magnetotransport in Pr0.5Ca0.5Mn0.99Cr0.01O3

R. Mahendiran, A. Maignan, M. Hervieu, C. Martin, and B. Raveau

Appl. Phys. Lett. 77, 1517 (2000); http://dx.doi.org/10.1063/1.1290726 (3 pages) | Cited 7 times

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Pr0.5Ca0.5Mn0.99Cr0.01O3 shows several anomalous properties: Field-cooled (FC) resistivity at 10 K is several orders of magnitude lower than the zero-field cooled (ZFC) value, magnetoresistance isotherm at 125 K is asymmetric about the origin (H = 0 T) and resistivity and magnetization are hysteretic between field cooling and field warming. No clear difference between ZFC and FC magnetization up to 5 T was found. We suggest that nanometric-size ferromagnetic clusters coexist with the charge-ordered antiferromagnetic matrix in H = 0 T and the matrix breaks into domains upon field cooling which leads to low resistivity. © 2000 American Institute of Physics.
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75.47.Gk Colossal magnetoresistance
75.50.Dd Nonmetallic ferromagnetic materials
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
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Dielectric enhancement and Maxwell–Wagner effects in ferroelectric superlattice structures

D. O’Neill, R. M. Bowman, and J. M. Gregg

Appl. Phys. Lett. 77, 1520 (2000); http://dx.doi.org/10.1063/1.1290691 (3 pages) | Cited 85 times

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In an attempt to reproduce the functional properties associated with relaxor electroceramics, pulsed laser deposition has been used to fabricate thin-film capacitor structures in which the dielectric layer is composed of a superlattice of Ba0.8Sr0.2TiO3 and Ba0.2Sr0.8TiO3. The properties of the capacitors were investigated as a function of superlattice periodicity. The dielectric constant was significantly enhanced at stacking periodicities of a few unit cells, consistent with relaxor behavior. However, enhancement in dielectric constant was generally associated with high dielectric loss. Analysis of the imaginary permittivity as a function of frequency shows that fine-scale superlattices conform to Maxwell–Wagner behavior. This suggests that the observed enhancement of the real part of the dielectric constant is an artifact produced by carrier migration to interfaces within the dielectric. A comparison of this data with that already published on dielectric superlattices suggests that previous claims of an enhancement in dielectric constant may also be attributed to the Maxwell–Wagner effect. © 2000 American Institute of Physics.
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77.55.-g Dielectric thin films
77.84.Ek Niobates and tantalates
77.84.Cg PZT ceramics and other titanates
84.32.Tt Capacitors
85.50.-n Dielectric, ferroelectric, and piezoelectric devices
77.80.-e Ferroelectricity and antiferroelectricity
81.15.Fg Pulsed laser ablation deposition
77.22.Gm Dielectric loss and relaxation
77.22.Ch Permittivity (dielectric function)

Structural and dielectric properties of epitaxial Ba1−xSrxTiO3/Bi4Ti3O12/ZrO2 heterostructures grown on silicon

C. L. Canedy, S. Aggarwal, Hao Li, T. Venkatesan, R. Ramesh, F. W. Van Keuls, R. R. Romanofsky, and F. A. Miranda

Appl. Phys. Lett. 77, 1523 (2000); http://dx.doi.org/10.1063/1.1290724 (3 pages) | Cited 18 times

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We report on the dielectric properties of an epitaxial heterostructure comprised of Ba1−xSrxTiO3, Bi4Ti3O12, and (ZrO2)0.91(Y2O3)0.09 grown on silicon substrates for potential use in microwave devices. Careful x-ray analysis indicates crystallographic alignment of all layers and transmission electron microscopy and Auger analysis reveals high quality epitaxy with minimal interdiffusion. The viability of using such heterostructures in actual microwave devices was assessed by incorporating the films in a coupled microstripline phase shifter design. The phase shifter devices, operating in the Ku band, had losses of less than 4 dB with a maximum phase shift of nearly 40° at 40 V. We compare this performance with a (Ba, Sr)TIO3/MgO phase shifter. These results presented represent significant progress towards integrating ferroelectric films with conventional silicon technology. © 2000 American Institute of Physics.
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77.55.-g Dielectric thin films
77.84.Ek Niobates and tantalates
77.84.Cg PZT ceramics and other titanates
68.55.-a Thin film structure and morphology
84.40.Az Waveguides, transmission lines, striplines
81.15.Fg Pulsed laser ablation deposition

Study of the electronic conduction mechanism in Nb-doped SrTiO3 thin films with Ir and Pt electrodes

Sundararaman Gopalan, Venkat Balu, Jian-Hung Lee, Jeong Hee-Han, and Jack C. Lee

Appl. Phys. Lett. 77, 1526 (2000); http://dx.doi.org/10.1063/1.1308274 (3 pages) | Cited 11 times

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Strontium titanate (ST) is an attractive material for dynamic random access memory applications. Doping ST with other elements such as Nb has been shown to reduce problems such as frequency dispersion and resistance degradation associated with this material. In this report, we study the electronic conduction mechanism in Nb-doped ST films [Sr(Ti1−xNbxO3)] with Ir and Pt electrodes. Film thicknesses were of the order of 35–40 nm. It was found that the “true” leakage current satisfied both Schottky and Frenkel–Poole conduction mechanism equations for both positive and negative polarities. The effect of Nb content (x = 0, 0.001, 0.01, and 0.05) on the barrier heights using both the mechanisms were determined. It was found that the barrier height decreased with increased Nb content, which was correlated with the increase in leakage current. © 2000 American Institute of Physics.
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77.84.Ek Niobates and tantalates
77.84.Cg PZT ceramics and other titanates
73.40.Rw Metal-insulator-metal structures
77.55.-g Dielectric thin films
73.50.Fq High-field and nonlinear effects
73.30.+y Surface double layers, Schottky barriers, and work functions

Direct observations of electric field-induced domain boundary cracking in 〈001〉 oriented piezoelectric Pb(Mg1/3Nb2/3)O3–PbTiO3 single crystal

X. Tan, Z. Xu, J. K. Shang, and P. Han

Appl. Phys. Lett. 77, 1529 (2000); http://dx.doi.org/10.1063/1.1308060 (3 pages) | Cited 37 times

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In situ transmission electron microscopy study of electric field-induced cracking has been conducted on 〈001〉 oriented 0.65Pb(Mg1/3Nb2/3)O3–0.35PbTiO3 crystal. Fracture of the 90° domain boundary was directly observed in the tetragonal area under both static and alternating electric fields. The field strength required to induce domain boundary cracking was lower for alternating electric field than for static field. © 2000 American Institute of Physics.
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77.84.Ek Niobates and tantalates
77.84.Cg PZT ceramics and other titanates
77.65.-j Piezoelectricity and electromechanical effects
77.80.Dj Domain structure; hysteresis
62.20.M- Structural failure of materials

Thickness dependence of stress in lead titanate thin films deposited on Pt-coated Si

Desheng Fu, Takeshi Ogawa, Hisao Suzuki, and Kenji Ishikawa

Appl. Phys. Lett. 77, 1532 (2000); http://dx.doi.org/10.1063/1.1308061 (3 pages) | Cited 28 times

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Thickness dependence of the soft mode E(1TO) of tetraganol lead titanate thin film, deposited on Pt-coated Si by a chemical solution deposition, was determined with Raman scattering measurements. A downshift of the soft mode was attributed to the residual stress in the thin film, which was estimated in the range of 1.3–2.6 GPa, corresponding to film thickness of 400–50 nm. The variation of the clamped dielectric constants determined by observed mode frequencies was found to agree with the prediction of stress dependence of dielectric constants by Devonshire theory. © 2000 American Institute of Physics.
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77.84.Ek Niobates and tantalates
77.84.Cg PZT ceramics and other titanates
68.60.Bs Mechanical and acoustical properties
77.55.-g Dielectric thin films
78.30.Hv Other nonmetallic inorganics
63.70.+h Statistical mechanics of lattice vibrations and displacive phase transitions
81.15.Lm Liquid phase epitaxy; deposition from liquid phases (melts, solutions, and surface layers on liquids)
81.05.Je Ceramics and refractories (including borides, carbides, hydrides, nitrides, oxides, and silicides)
77.22.Ch Permittivity (dielectric function)
78.66.Nk Insulators

Orientation dependence of the ferroelectric and piezoelectric behavior of Ba(Ti1−xZrx)O3 single crystals

Zhi Yu, Ruyan Guo, and A. S. Bhalla

Appl. Phys. Lett. 77, 1535 (2000); http://dx.doi.org/10.1063/1.1308276 (3 pages) | Cited 58 times

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The dielectric properties, ferroelectric polarization, and piezoelectric strain behavior of Ba(Ti1−xZrx)O3 (x = 0.05 and 0.08) single crystals are studied for different crystal orientations. The study of dielectric properties suggests that the three phase transitions in BaTiO3 are pinched by substituting Ti with Zr but the anisotropic dielectric behavior is present in crystals of various orientations. The remnant polarization and strain behavior are also studied for different crystal directions. Higher strain and lower polarization are obtained along the pseudocubic 〈001〉 direction compared to those observed along the 〈110〉 and 〈111〉 directions in both orthorhombic (x = 0.05) and rhombohedral (x = 0.08) phase. The results for the 〈001〉 oriented orthorhombic crystals with x = 0.05 indicate the highest strain level (∼0.25%) with almost hysteresis-free behavior and high piezoelectric coefficient (d33 = 500 pC/N). © 2000 American Institute of Physics.
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77.84.Ek Niobates and tantalates
77.84.Cg PZT ceramics and other titanates
77.65.Bn Piezoelectric and electrostrictive constants
77.80.B- Phase transitions and Curie point
77.22.Ej Polarization and depolarization
77.80.Dj Domain structure; hysteresis
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Electron saturation velocity of GaInP deduced in a GaInP/GaAs/GaInP double heterojunction bipolar transistor

Yue-Ming Hsin, Shih-Tzung Hsu, and Chen-Chung Fan

Appl. Phys. Lett. 77, 1538 (2000); http://dx.doi.org/10.1063/1.1290602 (2 pages) | Cited 2 times

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GaInP/GaAs/GaInP double heterojunction bipolar transistors have been fabricated for the study of electron saturation velocity (vsat) in GaInP. The necessary composite design at the base–collector junction, which effectively reduces the conduction band spike and avoids the premature Kirk effect, enables us to use the Kirk effect to study vsat. The deduced electron saturation velocity in GaInP is ∼ 5×106 cm/sec. © 2000 American Institute of Physics.
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85.30.Pq Bipolar transistors
85.30.De Semiconductor-device characterization, design, and modeling
73.50.Fq High-field and nonlinear effects
73.61.Ey III-V semiconductors

Photovoltaic heterostructure devices made of sequentially adsorbed poly(phenylene vinylene) and functionalized C60

H. Mattoussi, M. F. Rubner, F. Zhou, J. Kumar, S. K. Tripathy, and L. Y. Chiang

Appl. Phys. Lett. 77, 1540 (2000); http://dx.doi.org/10.1063/1.1290723 (3 pages) | Cited 35 times

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We report on the preparation and characterization of rectifying photovoltaic heterostructure devices made of poly(phenylene vinylene), PPV, and C60. The heterojunctions were built from solution using the technique of layer-by-layer sequential adsorption. This technique permits one to control the heterostructure at the molecular scale. Upon illumination with a laser beam, the devices showed large photoresponses (current and voltage) that resulted from a photoinduced electron transfer between the PPV (donor layer) and the C60 (acceptor layer). The photocurrent was found to increase with the laser power and with the photon energy of the incident radiation. Also, a constant high photovoltage response of ∼700–800 mV was measured. Analysis of the time dependence of the photocurrent rise and decay, when the device was illuminated with a modulated square wave signal (chopped laser beam), permitted us to draw an analogy between the present heterojunction and a circuit made of a capacitor and a resistance in series. © 2000 American Institute of Physics.
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73.61.Ph Polymers; organic compounds
73.61.Wp Fullerenes and related materials
85.60.-q Optoelectronic devices
73.50.Pz Photoconduction and photovoltaic effects

GaNAs resonant-cavity avalanche photodiode operating at 1.064 μm

G. S. Kinsey, D. W. Gotthold, A. L. Holmes, and J. C. Campbell

Appl. Phys. Lett. 77, 1543 (2000); http://dx.doi.org/10.1063/1.1308272 (2 pages) | Cited 14 times

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A resonant-cavity avalanche photodiode using a GaNAs absorption region grown on GaAs has been demonstrated. The peak quantum efficiency was 59% at 1.064 μm with a full width at half maximum of 11 nm. The absorption coefficient was determined to be α = 9×103/cm at this wavelength. The devices exhibited gain up to 100 at a low breakdown voltage of 13 V. © 2000 American Institute of Physics.
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85.60.Dw Photodiodes; phototransistors; photoresistors

Parametric amplification in a torsional microresonator

Dustin W. Carr, Stephane Evoy, Lidija Sekaric, H. G. Craighead, and J. M. Parpia

Appl. Phys. Lett. 77, 1545 (2000); http://dx.doi.org/10.1063/1.1308270 (3 pages) | Cited 11 times

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We observe parametric amplification in a torsional micron-scale mechanical resonator. An applied voltage is used to make a dynamic change to the torsional spring constant. Oscillating the spring constant at twice the resonant frequency results in a phase dependent amplification of the resonant motion. Our results agree well with the theory of parametric amplification. By taking swept frequency measurements, we observe interesting structure in the resonant response curves. © 2000 American Institute of Physics.
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07.10.Cm Micromechanical devices and systems

Staircase band gap Si1−xGex/Si photodetectors

Zhiyun Lo, Ruolian Jiang, Youdou Zheng, Lan Zang, Zhizhong Chen, Shunming Zhu, Xuemei Cheng, and Xiabing Liu

Appl. Phys. Lett. 77, 1548 (2000); http://dx.doi.org/10.1063/1.1286958 (3 pages) | Cited 2 times

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We fabricated Si1−xGex/Si photodetectors by using a staircase band gap Si1−xGex/Si structure. These devices exhibit a high optical response with a peak responsive wavelength at 0.96 μm and a responsivity of 27.8 A/W at −5 V bias. Excellent electrical characteristics evidenced by good diode rectification are also demonstrated. The dark current density is 0.1 pA/μm2 at −2 V bias, and the breakdown voltage is −27 V. The high response is explained as the result of a staircase band gap by theoretical analysis. © 2000 American Institute of Physics.
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85.60.Gz Photodetectors (including infrared and CCD detectors)
07.57.Kp Bolometers; infrared, submillimeter wave, microwave, and radiowave receivers and detectors
73.40.Ei Rectification
71.20.Nr Semiconductor compounds
85.60.Dw Photodiodes; phototransistors; photoresistors
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Comparison of penetration depth between two-photon excitation and single-photon excitation in imaging through turbid tissue media

Min Gu, Xiaosong Gan, Aernout Kisteman, and Ming Gun Xu

Appl. Phys. Lett. 77, 1551 (2000); http://dx.doi.org/10.1063/1.1308059 (3 pages) | Cited 14 times

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We show, both theoretically and experimentally, that for a turbid tissue medium where Mie scattering is dominant, multiple scattering not only reduces the illumination power in the forward direction but also exhibits an anisotropic distribution of scattered photons. Thus, a signal level under two-photon excitation drops much faster than that under single-photon excitation although image resolution is much higher in the former case. As a result, the penetration depth under two-photon excitation is limited by the strength of two-photon fluorescence and is not necessarily larger than that under single-photon excitation. © 2000 American Institute of Physics.
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87.63.L- Visual imaging
87.50.W- Optical/infrared radiation effects
42.25.Fx Diffraction and scattering
87.64.Cc Scattering of visible, uv, and infrared radiation
42.30.Va Image forming and processing
42.25.Dd Wave propagation in random media
42.62.Be Biological and medical applications
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Influence of adsorbates on the surface conductivity of chemical vapor deposition diamond

M. Szameitat, X. Jiang, and W. Beyer

Appl. Phys. Lett. 77, 1554 (2000); http://dx.doi.org/10.1063/1.1290725 (3 pages) | Cited 20 times

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To investigate the mechanism of the diamond surface conductivity, temper experiments have been performed on chemical-vapor-deposited (CVD) diamond films under vacuum conditions. The surface conductivity of these films was measured as a function of temperature and in contact with different gas atmospheres. These results were compared with those obtained by gas evolution experiments performed on CVD diamond samples of the same kind, demonstrating the possible role of CO adsorption to the surface conductivity. © 2000 American Institute of Physics.
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73.61.Cw Elemental semiconductors
73.25.+i Surface conductivity and carrier phenomena
68.03.Fg Evaporation and condensation of liquids
68.43.Mn Adsorption kinetics

Imaging soft samples in liquid with tuning fork based shear force microscopy

W. H. J. Rensen, N. F. van Hulst, and S. B. Kämmer

Appl. Phys. Lett. 77, 1557 (2000); http://dx.doi.org/10.1063/1.1308058 (3 pages) | Cited 35 times

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We present a study of the dynamic behavior of tuning forks and the application of tuning fork based shear force microscopy on soft samples in liquid. A shift in resonance frequency and a recovery of the tip vibration amplitude have been observed upon immersion into liquid. Conservation of the vibration mode is confirmed by both direct stroboscopic observation and by detection of the tip vibration amplitude of the tuning fork. Thanks to the partial recovery of the Q factor upon complete immersion into liquid, it is possible to obtain high-resolution images on soft samples in liquid. This opens a new domain of applications for tuning fork based near-field scanning optical microscopes. © 2000 American Institute of Physics.
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07.79.Fc Near-field scanning optical microscopes
87.64.mt Near-field scanning
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